Among prior art image-reading devices (sometimes termed “image scanners” or simply “scanners”), there are those in which an optical unit, which includes optical elements such as mirrors and lenses which guide light that is reflected from a document to a photoelectric conversion device, is moved along a document in order to obtain image data from the document. The optical unit is incorporated into a carriage and a driving mechanism causes the carriage to move. For example, a driving belt such as a timing belt or the like may be moved back and forth by the output rotation of a driving motor, with the carriage being connected to the driving belt. In addition, to guide the movement of the carriage, a guide rod extends in the direction of movement (i.e., in the lengthwise direction) and is connected to the carriage.
The carriage is molded of plastic (i.e., a synthetic resin) and is formed into a suitable shape. A light source, photoelectric conversion device and optical elements are incorporated at predetermined positions on the carriage.
In order to improve portability and storability of an image-reading device, there is market demand to make the carriage which supports the optical unit as thin as possible, thereby enabling the image-reading device itself to be thinner. However, the carriage must be longer than the width of the original document to be scanned. Hence, in order to be able to acquire images from, for example, A4 size paper, the carriage must be about 220 mm in length. Designing the carriage to have a sufficient thickness in order to properly support such a long carriage becomes an impediment to the objective of making the carriage thinner. In other words, in order to achieve sufficient thinness of the image-reading device, it is necessary to provide a carriage that is thinner than deemed advisable in terms of keeping the carriage from undergoing strain. Such strain, or bowing of the carriage, will jeopardize the accuracy in positioning of the optical elements carried by the carriage. Additionally, there is a concern that the length of the light paths at the edge and at the center of the carriage may become different, so that the image data acquired is not faithful to the document being scanned. This will cause images, obtained by reproducing the image data, to be unclear.